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Tom Riddle as Epic PC in D&D 4e

The signature abilities of Tom are the killing curse (instakill), the crucio curse (debuffing pain) and the imperius curse (dominate). He can teleport and fly and can survive death.

The Archlich epic destiny (requiring an arcane class) satisfies the death and survival elements.

The "Hexer" warlock PP suits the feeling of Tom Riddle quite neatly:

You have delved deeply into the intricacies of crafting curses. You have discovered that greater effects can be wrung from a well-placed curse than most warlocks realize. When you place a hex, your foes reel from the power of your curse. You have learned to weave your spells with your curses and your curses with your spells. Your body and mind resonate with the damning curses you articulate. When you speak, all who hear your words flinch.

And warlock is a far superior fit than wizard.

Thus, the class is warlock, class feature: Dark Pact (which suggests his research into the dark arts, rather than any specific bargains he made)

You sequester a portion of a creature’s life force for emergencies. The recipient of this ritual (which can be you) spends one healing surge. The next time the recipient is reduced to negative hit points equal to his bloodied value and would ordinarily die, he instead regains hit points equal to his healing surge value.

Which is a level 20 ritual. Also as part of this is the stipulation to the GM that the ritual of retrieval works first. As this takes the player out of the adventure, it is a house rule that may or may not be accepted, depending.

You perform this ritual at a specific safe location, attuning the site so that your body returns to it if you are killed. On the first round after you die, your body and all your possessions burn away to ash, reforming (still dead) in the locale you attune with this ritual. If the attuned location has been destroyed or altered in a way that prevents your body’s return, the ritual fails.

The absolute earliest time Tom could achieve this, in cooperation with a GM, would be to find a scroll of Soulguard. Kinda boring, but it just works.

The cheesiest, earliest way Tom could achieve this would be through the artificer multiclass, taking "Master crafter" (to add int bonus to possible levels of magic items made), persuade the GM (through a quest or two) to be able to make a Horreb Ritual cube at effective level 20 (it's level 18, but for it to be able to contain any ritual at 18 is just silly. It's a maguffin and should be treated as such.) In order to do this effective 20 ritual, as a warlock (primary ability cha, secondary int), assuming 16 cha /16 int would be Level 15 (or level 13, depending if the GM was feeling merciful and allowing a ritualist's lectern to stack.)

The earliest non-cheesy, no GM permission would be level 18 from ritualist's lectern.

Therefore, as we can get the horcrux through rituals, we need to consider what class fits the style of HP wizardry best.

The general style is in single-person attacks, with very rare multi-person attacks (like fiendfyre). We see bolts of magic, rather than fields.

Tom is remembered for three powers, the unforgivable curses: death, pain, and dominate. Death is easy. Pain should be a nasty debuff, and dominate is tricky.

Wizards do not have many domination spells, their style of At-Wills are D&D wizardry rather than potter wizardry. While we will consider them, they are not the first choice.

The best dominate powers in order are:

Psion 23 Sudden Control - AT WILL (but for true dominate, you need to augment)

Warlock 15 Hellfire Soul - Daily, but if you drop someone with this, they're dominated until the end of the encounter

Psion 23 Daggers of Pain (vulnerable to psychic is great, though not at all what crucio is about. Is still quite effective.

Warlock 9 Ring of Pain has an echo effect that's not in-style, but effective.

Psion 1 Mind thrust penalty to attacks suggests the pain, but it doesn't have save ends

The wizard debuffs don't suit the tone, in their mechanics.
Unfortunately, WoTC hasn't produced a book of evil cheese for 4e yet, and so the necessary sadist components are missing. Psion/warlock feels about right, here.

The best "You. Die." powers are... completely absent from PCs in this edition. The best bet is to cheese up a damage spell, and reflavour it with green die and sparkly lights. (Sorry, but PCs don't get instakills.)

From this, Warlock, Psion, or a hyrbid would work very well for Tommy, depending. The PP would support whichever of the primary powers you wanted to go after, and ED could be basically anything that supports those three powers, though archlich is one of the better ones.

He will be a Solo, and to keep the fight interesting, we'll apply the Lawbreaker mechanics, so the PCs can't trivially stunlock him. For purposes of this discussion, we won't build the encounter as well, though with any solo, 3-4 different locations are quite important to keep things interesting.

Now, as monster classes go, he is easily a Controller, Leader.

With that said, let's first adapt the magical abilities from the wiki into interesting powers for 4e monsters:

Magic Mastery: "Pushed the limits of magic farther than they have ever been pushed" and "wizard alive, and that even his most powerful protective spells and charms were unlikely to be effective if Voldemort returned to full power. "

Part of this will be the "lawbreaker" support, as he "warps the rules of reality" , but the "powerful protective charms" suggests some way of disabling player items. (extremely mean, but interesting as part of the climax of a campaign.)

"In the films, Voldemort has shown the ability to contain spells and release them in a blast that resembles a shock wave."

This reads like a wonderful "Immediate interrupt" for damage redirection.

"Voldemort was also a masterful practitioner of all three Unforgivable Curses, with a special affinity for the Killing Curse, having murdered enough people to create an entire army of Inferi. Ironically, this would be his final downfall."

This gives us the primary standard actions.

Infiri as shambling zombie corpses make great minions that auto-spawn around the dark lord.

"Avada Kedavra" as killing curse will be OMG (a technical term, actually) necrotic and psychic damage to a single target. Insta-kills are boring and, as HP in many ways represent "dodging" and expending energy, the massive damage makes sense. As an alternative, it could just be a "kill a bloodied target" which is... harsh.

"Crucio" as a pain curse is well-implemented. Ongoing save ends psychic damage with a nasty penalty to hit.

And "Imperio" is a dominate.

The wiki lists: "Lightning spells: Voldemort cast a powerful lightning spell up in the air when he tested the Elder Wand after stealing it from Dumbledore's tomb." which gives us a nice encounter power, perhaps part of the lawbreaker progression, or an aura.

"Master duellist" is... functionally a description of a solo

"Charms master" suggests invisibility and item teleportation.

"Fire spells" is... another elemental type. This theme suggests that one of these spells should be used to signal each stage of the fight.

"Apparation" means he'll have a teleport speed.

"Levicorpus" means he'll have forced movement, probably as a minor.

Flight... does what it says on the tin.

The Horcruxes aren't really relevant, as the encounter will assume that the epic level PCs have disposed of them all already. If they haven't, the epic will have an "on death, not really dead" effect.

Transfiguration master (does this list ever end?): not hugely interesting, but will be part of the lawbreaker thing.

Thoughts on the killing curse: Instant death is boring, therefore instant death is bad. The threat of instant death, is good. For this, we'll be stealing the beholder death ray:

Rough mechanics

We start with the known: we want to challenge a group of 5 30th level PCs with the big bad of an entire campaign. (As an aside, it's quite feasible to flavour the 4e classes such to fit into the potterverse).

The highest level creatures in the compendium are level 36, so Tommy will be 36, himself.

Speed 6, Fly 8 (Hover), Teleport 10 gives us someone who is most annoying in movement speeds and a nice moving fight.

Roughly speaking, he's got 3 "sthicks" beyond the unforgivable curses, which will be his bread and butter.

Fire, Lightning, and Charms.

This gives us 3 "modes" for the combat to progress through, modifying the primary abilities.
(I wish I could give credit, but I can't seem to find the link. Inspired by a recent encounter featuring a blue half-dragon solo with 3 modes. )

Mastery of Dark magic: Tom may perform up to 2 charms or unforgivable curses at two different targets.

Minor:

Summon minions: If Tom has less than 6 Inferi alive within 6 squares of him, he may summon enough to bring the total up to 6.

Triggered:

Reflect spells, immediate interrupt, may retarget one ranged or area burst attack.

Summary of base monster: He's nasty, but never takes a party member out of combat without a "warmup" first. This keeps the threat of death and domination high without having party members sit out stunlocked. The -5 to attacks of crucio is effectively stunned, but without the nasty "just roll a die" stun. He's got his signature "I'm awesome at magic" and "I'm awesome at the unforgivable curses" going, with a nice background of "mmm, minions" to keep the fight interesting.

At 1300 HP (roughly), and at 43-141 damage per round from PCs (cite) call it 92 damage per PC per round, 14 PC rounds of combat, not counting the negative status effects. That feels about right, though HP will have to be adjusted to suit party composition.

Now, to add in even more flavour, and to mix up tactics, we'll take a page from here. Functionally, every third of his HP, Tommy will switch flavours, to represent increasing desperation. The swap will be an aura power, a different standard action, and some kind of minor/interrupt.

The first third is the "Charms" school. Charms are "enchantments of objects that cause unusual behaviour"

Minor action: "Severing charm" Tom causes an item that is not a weapon, implement, armor, or neck slot item to be "damaged" (save ends.) A damaged item is not useful in combat.

Standard action: "Confusing charm" Target must choose targets randomly for any power they use. (Save ends)

Aura: Legilimency, range 5, targets must roll twice and take the lower of two rolls when attacking Tom.

This features mainly "annoying" attacks that heighten expectations. When Tom is reduced to 2/3rds hitpoints, he attacks everyone on the battlefield (including his minions) with Electrified Vengeance.

Electrified Vengeance: All creatures in a close burst 15, 15d10+60 lightning and thunder damage.

Forceful curses: All of Tom's curses may slide a creature 4 squares in addition to their other effects.

Aura: Lightning ring: starting 4 squares away from Tom, and extending another 4 squares is a lightning ring, which causes 30 ongoing lightning damage (save ends) to any creature that starts their turn in the aura. And he will keep sliding them into that aura, and teleporting where it will do the most harm.

When Tom is reduced to 1/3rds hitpoints, he then switches to fire. As a free action, he drops the lightning aura and conjures 4 fiendfyres within range 10, area burst 1. Creatures that start their turns within the zone take 50 fire damage. (Ow, it's hot.)

Minor action: move a fiendfyre zone up to 6 squares.

Standard action: Gout of flames, Each area of fiendfyre attacks all creatures within 5 squares with 4d12+31 fire damage.

Final Thoughts

Obviously, the above creature needs to be tested before being thrown as the final monster of an epic campaign. I haven't included a formal stat block because all the numbers will need to be tweaked by the GM to fit the party. This was mainly an exercise in how to create a themed solo that will keep the players interested in 4th edition.

The Horcruxes are a specific form of a The Lich’s Phylactery. Voldermort's version is in multiple vessels, all of which must be destroyed for him to be truly dead.

As for his powers, he is basically a powerful wizard who's good at just about everything. D&D Magic is not the same as Harry Potter Magic, which looks like it works more like GURPS Magic than the Vancian system of D&D.

If it is D&D 4e, just make him a 30th level wizard with the feat of talking to snakes plus maybe a cross-class feat, and pick a few psionic powers to represent his skill at Legilimens and Occlumens. Other editions make him 20th+ level and able to cast multiple 9th level spells.

He has two disadvantages or curses (in older D&D parlance). The first is he has a two-way mind reading connection to Harry Potter, and second his Legilimen abilities can be blocked by those who have intense feeling of love towards family or individuals. It's how Snape was able to be Dumbledore's spy for so long.

Tom Riddle as Character in 3.5

"The process makes the part of the soul remaining in the witch or wizard unstable. If the maker's physical body is later destroyed, he or she will live on in non-corporeal form, although there are methods of regaining physical form."

and

"The fragments of a person's soul within a Horcrux can think for themselves and have certain magical abilities, including the ability to influence those in their vicinity"

The base spell of interest, here is Magic Jar. Sends your soul into an object which can then go possess things.

This gives us 2 problems.

Magic Jar isn't unlimited duration

Your initial body is lifeless.

One other route is through the spell Clone, which creates bodily backups in case of death. This, however, strays from the idea of the insubstantial, possessing Horcrux.

The third route is through psionics, the power Mind Seed, suitably delayed and imbued into an item also simulates the gradual possession.

As the lowest-level option, the rest of this answer will explore Magic Jar.
Some aspects of it will be taken from this list of 3.5 feats.

In order to get Magic Jar with a permanent duration, it must be enchanted into an item, this is handy due to the other requirement of gradual control: the item can cast magic jar and manipulate its wearer according to the requirements of the creator, Tom Riddle.

In order to perform Soul Duplication, three things are needed. The ability to manifest the power "Fission", psionic-magic separation, and a thought bottle.

Fission duplicates the manifester into two creatures, one at level-1 and one at level-2. The level-2 creature uses the thought bottle to restore the lost levels. If the one at level-2 enters the magic jar intelligent item, its body becomes lifeless, and as such, satisfies the "dead" criterion for Fission, which immediately ends.

With psionic-magic separation, the line "All powers affecting a fissioned creature, either the original or the duplicate, end when the fission ends. All damage, including hit point damage, ability damage, ability drain, and ability burn damage, is added together." happily doesn't apply with psionic-magic separation (much easier to justify in an NPC).

Thereby the caster has split their soul (in the literal and figurative sense) and stored a portion of it in an object through magic jar. This action, coming from a PC, functionally creates an immediate NPC, but that's another problem that's outside the scope of this answer.

In order to cast from both psionic and sor/wis lists, we turn to the Erudite (Complete Psionic optional Psion) and the Mind's Eye spell-to-power ability, with arbitrary amounts of cheese to raise caster level and effective caster level to perform this at the desired time. Given that sacrifices are a great way to get limited wish and itemXP, this works quite well in that synergy.

Avada Kedavra is not power-word kill, because power-word kill is kind of pathetic. Stop heart, (Cleric 4, Sor/Wis 5) Book of Vile Cheese is far more appropriate, with arbitrary cheese applicable in the normal fashion.

Crucio is perfectly represented in the BoV Cheese spell "wrack".

And Imperius is represented in any of the many domination spells out there.